Tagron had the right answer, but to expand.....
The point made, was that you don't need to "clear customs" outbound. However, many people like a last stop before committing to over water, and full tanks are always good. Part of the enjoyment of going foreign is getting back £50 or so from HMRC for doing it!
So if Arclite wants recommendation, I'd definitely go for Rochester. A straight line from there to Cap Griz-Nez gives the shortest crossing of 21nm or so (then turn left or right depending where you want to go). The advantage IMHO of starting at Rochester is that you can climb to a decent height for the crossing between Rochester and Folkestone, so have a great gliding range by the time you reach the coast (not true if you take-off from Lydd unless you circle).
Whilst an outbound GAR is not mandatory, it's simple enough to do at the same time as you file your flight plan (which is). Even easier if you have Skydemon, which will help you through it. The advantage of submitting outbound and inbound as you are leaving, lets people know who is on board and presumably matches up those going out and returning.
And yes, I have used both EGTO & EGMD for "clearance" and the only advantage I can see for Lydd would be if you prefer to fill in a paper flight plan and wait for it to be submitted. If you are into online, then you can do that from anywhere (including computer in Rochester's tower). If going at very wet periods of the year, check with Rochester re runway condition - they haven't yet got permission for their hard runway. I've discounted Southend due to cost, controlled airspace, and distance from coast.
Is that a decent answer?